Considering the numerous very archaic features which the general structure of this group displays, it is surprising to find how typically mammalian they are in certain other peculiarities. The mammalian diaphragm, one of the distinguishing features of the class, is perfectly normal in the Monotremata. The alimentary canal shows no great divergences from the normal structure. The stomach is almost globular, with a projecting pyloric region in _Ornithorhynchus_; the intestine is divided into a "small" and "large" intestine by a slender caecum. The liver has the subdivisions that this organ usually shows in the Mammalia. However, the presence of the ventral mesentery and of the abdominal vein in _Echidna_ and _Ornithorhynchus_ has already been mentioned as a distinctive character. The peculiar and apparently partly primitive valve of the right ventricle has been described above (see p. 66). The brain is in most respects mammalian in its characters, but naturally shows some important differences. Dr. Elliot Smith, who has most recently studied this question,[57] is of opinion that the size of the cerebral hemispheres is not at all reptilian; indeed, it "greatly exceeds that of {110} many other mammals." In _Echidna_, too, but not in _Ornithorhynchus_, the hemispheres are well convoluted, though the arrangement of these convolutions cannot be brought into line with what is known concerning the convolutions upon the hemispheres of other mammals. It had been stated that in these animals, at least in _Echidna_, there were only two optic lobes, as in lower vertebrates, instead of the mammalian four. The late Sir W. H. Flower set this matter at rest,[58] and showed that _Echidna_ was in this respect typically mammalian. The absence of the corpus callosum is one of the principal features separating the Monotremes from other mammals. The Monotremata are represented to-day by two types, _Ornithorhynchus_ and _Echidna_, which are no doubt worthy of being placed in separate families. Fossil remains of the group (apart from the problematical Multituberculata) are only known from Pleistocene times in Australia, and consist of the bones of a large species of _Echidna_, and some fragments of _Ornithorhynchus_, indicating a smaller animal than the living Platypus. [Illustration] FIG. 53.--Brain of _Echidna aculeata_, dorsal view. (Nat. size.) (From Parker and Haswell's _Zoology_.)